Exhaust gas temp

[Justinp551]

So the discussion came up amongst a few fellow flight instructors about leaning engines and temps of exhaust gas temps. Some quick background.

Mostly all general aviation engines arel air cooled and rely on a rich mixture on takeoff and climb to keep the cylinders from overheating. Now when you level off in cruise and have the power back a bit you lean the mixture(it's cockpit adjustable) to specific settings. As you lean the mixture the exhaust gas temps will increase to a peak and continued leaning will bring the temps back down. There is a long argued debate about lean of peak vs rich of peak operation in cruise. Without going out on a tangent on that subject...

What theory causes the egt to change with a different mixture? I am hearing evaporation, un burnt fuel pulling heat out, my theory is the different mixtures burn at different temps and or speeds.

Braaap and Tony D always seem to be a wealth of knowledge on the dynamics of engine operations and I know there is alot of other smart minds on here so...

Throw your ideas or theories out please

[Trevor]

My guess is the "Liquid" portion of the air/fuel vapor can carry more heat that the gaseuos portion. More heat is moved in a rich mixture out the exhaust by its fluid movement instead of being absorbed by the metal of the engine block, head, and exhaust manifold(s).

[Leon]

I'll try to briefly summarize this.

 

Exhaust gas temperature (EGT) depends on combustion temperatures. The hotter the mixture burns inside the cylinder, the hotter it will be coming out. Theoretically, combustion temperatures are at a maximum at stoichiometric, but realistically the maximum occurs slightly rich from peak because of the dissociation of Oxygen from the combustion products (CO₂, H₂O). Why temperature drops when rich or lean is described by the energy released caused by the chemical reactions between the fuel and air. Too little fuel (lean) and there is less energy contained within and more heat is transferred to the cylinder walls (no fuel evaporation or boundary layer), thus the lower temperature when it burns. Too much fuel, and combustion efficiency drops thus generating less heat.

 

I know, this is not too detailed, but it gets the point across without involving too much technical jargon.

 

Of course, this all assumes MBT timing and stable combustion. You can also change exhaust temps by varying spark timing, arguably moreso than by just varying AFR. EGT is increased when spark timing is retarded since you are giving the gasses in the cylinder less time to cool off before the exhaust valve opens.

[rsicard]

Being a 3 time owner of General Aviation aircraft, timing is fixed (not adjustable) by the magnetos. Flew cross-country flights in fuel injected Cessna 210's and fuel mixture was set to the lean side of peak EGT. This is important to get the most mileage out of a gallon of fuel. On the lean side of peak, fuel is reduced and power and EGT are slightly reduced. On the rich side of peak, not all the fuel admitted to the cylinders is not completely burnt and therefore applies fuel liquid cooling to the cylinders.

[Tony D]

Yes, the fuel cooling aspect has very little to nothing to do on the back end. If you assume static ignition timing the explanation Leon gave becomes easier to understand. As he states on the last paragraph, you can vary EGT by spark timing as well. But just assume everything equal and you can follow the explanation.

 

Leaner fuel mixes burn 'faster' so lean of peak is explained by the combustion being over when the valve opens.

Richer mixtures burn 'slower' so rich of peak is explained by combustion still continuing on into the manifold when the valve opens.

 

As you can see, each condition would require DIFFERENT spark timing to get the SAME EGT (and this IS possible) so it gets into almost a three-dimensional chart regarding rich/lean/and spark timing. (Moreso if you are in a vehicle with variable throttle position but this is aircraft....)

 

Once you get to a point where you start misfiring (lean or rich) it can throw the EGT's off depending on where the probe is as ignition in the exhaust manifold can occur and spike temperatures.

 

But basically when you run 'lean' they can spike to a point based on ignition timing (usually too low for burn rate anyway), and then leaner from that point gives Leon's explanation of less fuel input and less possible output.

[rsicard]

Timing in General Aviation Aircraft if FIXED! This throws out any assumptions of timing changes. Leaner mixtures DO NOT burn faster. There is simply less energy submitted to the cylinders past peak EGT and thus the less heat. There is no misfiring richer than peak in General Aviation Aircraft. There is misfiring if the mixture is set too LEAN. The EGT probes are always a specific dimension from the cylinder exhaust flange. Typically the aircraft monitor only ONE cylinder exhaust. There are some aftermarket EGT systems with a cylinder select switch for each cylinder. Very familiar with Gen Av Aircraft have flown MANY and worked on MANY.

[Leon]

Yes, rsicard is right, Tony has the burn rates backwards. A rich mixture burns faster and a lean mixture burns slower. Therefore, the slow burn of a lean mixture allows for more heat transfer to the cylinder walls. This is one of the reasons for a cooler EGT with a lean mixture.